Iron phosphate nanoparticle catalyst for direct oxidation of methane into formaldehyde: effect of surface redox and acid–base properties

Abstract

The effect of various iron phosphate and oxide catalysts on the direct oxidation of methane (CH₄) to formaldehyde (HCHO) with molecular oxygen (O₂) as the sole oxidant was studied using a fixed-bed flow reactor. Five crystalline iron-containing catalysts (FePO₄, Fe₃O₃(PO₄), Fe₄(P₂O₇)₃, Fe₂P₂O₇, and α-Fe₂O₃) with different iron coordination geometries, iron oxidation states, and Fe/P ratios were synthesized by the sol–gel method using malic acid or aspartic acid. The Fe/P molar ratio had a significant effect on the oxidation catalysis; CH₄ conversion increased with the Fe/P molar ratio, although the selectivity to HCHO decreased. Trigonal FePO₄ nanoparticles synthesized by the malic acid-aided method with an Fe/P molar ratio of 1/1 exhibited the highest activity for the selective formation of HCHO among the catalysts tested, including FePO₄ synthesized by a conventional method. Despite the much higher oxidizing ability of Fe₂O₃ than FePO₄, the oxidation of CH₄ using Fe₂O₃ resulted in the formation of only CO₂. In contrast, the temperature-programmed reaction of FePO₄ with CH₄ gave Fe₂P₂O₇ with the formation of HCHO as a primary product, and Fe₂P₂O₇ reacted with O₂ to regenerate FePO₄. Based on mechanistic studies including the catalyst effect, kinetics, pulse-reaction experiments, and IR spectroscopy, the bulk structural change between FePO₄ and Fe₂P₂O₇ is not involved during the catalysis and the surface redox and acid–base properties of FePO₄ are considered to play an important role in CH₄ oxidation with the structure preservation of bulk FePO₄. The combination of redox-active Lewis acidic iron sites and weakly-basic phosphate units likely contributes to the C–H activation of CH₄ and the suppression of complete oxidation to CO₂, respectively.